There has been practiced a conventional process of joining a contact for electric equipment such for example as an electromagnetic contactor (hereinafter referred to simply as a "contact") to a base. According to such a known process, a contact material is clad with a layer of silver, which is joined to a base as by spot welding.
Such a contact joining process utilizing heat caused by an electrical resistance is far more efficient in joining operation than other joining processes that use brazing, fusion welding, pressure welding, and the like. However, the prior joining process has resulted in unstable joined surfaces due to insufficient joining or an excessive amount of melted silver.
FIGS. 1(a) through 1(d) are schematic views illustrative of a joining process which relies on spot welding.
Designated in these figures at 1 is an upper electrode, 2 a lower electrode disposed in confronting relation to the upper electrode 1, 3 a contact material, 4 a layer of silver with which the contact material 3 is clad, 5 a contact that is composed of the joined contact material 3 and silver layer 4, 6 a base, 7 a joining interface, and 8 a mass of melted metal.
According to the spot welding, the contact 5 and the base 6 which are to be joined together are sandwiched between the electrodes 1, 2 as shown in FIG. 1(a), and are pressed and supplied with an electric current which flows therethrough. Part of the layer of silver is melted by the heat caused by an electrical resistance in a current path constituted by the contact material 3 and the silver layer 4. The melted silver partly flows from the joining interface to form a mass 8 of melted metal which joins the contact 5 and the base 6. The condition in which a proper amount of silver 4 that is attributable to stable joining is in quite a small range for the reasons described below.
The layer 4 of silver is employed to enable the contact material 3 and the base 6 to improve their joining, which would otherwise be quite poor. The silver layer 4 has a thickness which is normally about 10% of that of the contact 5. Silver layers which are available on the market in quantities are approximately 100 .mu.m thick. The joining area 7 is acceptable as shown in FIG. 1(c) when an optimum amount of such a silver layer 4 is melted from the joining interface.
If the amount of melted silver were too small, then the assembly would be joined as shown in FIG. 1(b). On the other hand, if the amount of melted silver were too large, then the contact assembly would be as shown in FIG. 1(d). In either case, the joined area would become poor. A good joined area such as shown in FIG. 1(c) might be achieved by making a strict selection of a pressure, a welding current, and an interval of time during which the current flows between the electrodes, welding conditions which are needed to melt a proper amount of silver in a thin layer 4 that is about 100 .mu.m thick. However, such welding conditions could vary with a primary voltage, a primary pressure, and wear of the electrodes, and it would be quite difficult to allow a proper amount of silver to be melted from its layer.
There has also been known a spot brazing process in which spot welding accompanies brazing filler metal and flux to enlarge and strengthen the area joined by the brazing filler metal. While the spot brazing process renders the joining interface somewhat better, it still falls short of accomplishing a fully stable and improved joining interface, and is also disadvantageous in that it requires additional materials such as brazing filler metal and flux.
Although the foregoing description has been directed to resistance welding as represented by spot welding, it has been as difficult to form a stable and improved joining interface with other joining processes.